As well known, magnetic recording apparatus are divided into those using a magnetic disk as the medium on which to record data and those using magnetic tape as such medium. Because the former type of recording apparatus using a magnetic disk (hereinafter referred to as magnetic disk drives) is prevailing, the following description focuses on magnetic disk drives as an example of the magnetic recording apparatus.
As the capacity enlargement of magnetic disk drives has been pursued for recent years, the fly height of the magnetic head has been lowered rapidly down to below 30 nm, and, consequently, there is increasing need for reliability in terms of resistance to sliding friction.
Also, there is strong need to enhance the data processing speed with more disk capacity. In particular, in a Redundant Array of Independent Disks (RAID) system, a magnetic disk drive that operates at a disk revolving speed of 10,000 rmp or higher is required.
In order to ensure the reliability of a magnetic disk drive, generally, a lubricant layer is formed on a carbon overcoat on the surface of a magnetic disk for use in the disk drive. As the main material of the lubricant layer, usually, fluoropolyether, which is a chemically stable fluorinated organic compound, is widely used.
Actually, in order to assure reliability of the magnetic disk drive, it is mandatory to efficiently preserve suitable lubricant distribution on the surface of said magnetic disk drive for long operating times. When magnetic disk drives operate, said disks revolve at a high speed and the lubricant might be spun off by the combined action of the air shear due to the air flow on the surface of the disk as the disk revolves, and of the centrifugal force directly exerted on the lubricant. As a consequence, it is often observed that the quantity of the lubricant on the surface of the disk gradually decreases. Also, evaporation phenomena of the lubricant into the atmosphere inside the magnetic drive may take place.
To overcome problems of the lubricant loss by being spun off during disk revolution and natural evaporation, approaches have heretofore been proposed. Thus, a method for restraining the lubricant from being spun off and evaporated has been proposed in which the adhesion force of the lubricant to the disk protecting layer is made stronger by increasing the polarity of the functional end-groups in the lubricant. Said polar end-groups are believed to improve adherence of the lubricant to the surface of the magnetic media.
Within this approach, fluoropolyether lubricants based on fluoropolyethers as the backbone and having hydroxyl functional groups as their end-groups have shown best performances.
A class of compounds which have been found to plenty fulfil requirements for being used as magnetic media lubricants are fluoropolyether derivatives comprising fluoropolyoxyalkylene chains and end-groups comprising multiple hydroxyl groups.
Such materials can be notably manufactured by reaction of epihalohydrins with perfluoropolyether derivatives having two hydroxyl end-groups (see Scheme 1 here below), as taught in TURRI, Stefano, et al. End group chemistry of fluoro-oligomers: highly selective syntheses of diepoxy, diallyl, and tetraol derivatives. (A) J. polym. sci, A, Polym. chem. 1996, vol. 34, p. 3263-3275. Scheme 1

Despite above sketched stoichiometry, side reactions are likely to occur during nucleophilic substitution on the epihalohydrin, involving e.g. reactions of oxirane ring with further PFPE hydroxyl derivatives, yielding materials comprising more than one PFPE chain block and/or different hydroxyl groups.
Similarly, reaction of perfluoropolyether derivatives having two hydroxyl end-groups with glycidol of formula:
as described in SCHICCHITANO, Massimo, et al. Synthesis and characterization of low-viscosity fluoropolyether-based segmented oligomers. Die Angewandte Makromolekulare Chemie. 1995, vol. 231, no. 1, p. 47-60, yields, in addition to the expected epoxy-substituted derivatives (which can be further converted in corresponding diols), a large range of side-products. As an example, PFPE hydroxyl derivatives can open the oxirane ring of the targeted compound, yielding materials comprising more than one PFPE chain block, and/or, more frequently, a further glycidol molecule can react with the epoxide ring of above mentioned targeted epoxy-substituted intermediate, so that different species are formed.
Mixtures obtained from processes of the prior art are thus generally complex compositions comprising unreacted precursors, targeted polyol derivatives and polymeric material comprising several PFPE chain blocks and/or several ex-glycidol molecules moieties, which require burdensome separation procedures.
Also, the broad molecular weight distribution of the starting perfluoropolyether diol mixture used for manufacturing corresponding tetraol derivatives by reaction with glycidol and/or epihalohydrins as above detailed makes it even more difficult to separate materials based on their respective volatility, as this parameter is both affected by the molecular weight and the functionality degree of the end-chains.
Complex purification procedures, based e.g. on supercritical carbon dioxide extraction techniques are thus required for purifying target material, so as to achieve the expected chemical structure and level of functionality at the end-groups.
Approaches of this type are described, for instance, in US 2004092406 (FUJI ELECTRIC CO LTD (JP)) May 13, 2004, in US 2003100454 (FUJI ELECTRIC CO LTD (JP)) May 29, 2003 and EP 1372141 A (HITACHI LTD (JP)) Dec. 17, 2003.
Due to the extreme wear conditions wherein said lubricants are to be used, in particular for hard disks lubrication, it is essential to be able to isolate compounds having well-defined functionality in end-groups and thus homogeneous behaviour in lubrication, adherence and resistance against evaporative loss.
The need was thus felt in the art for a process for the purification of hydroxyl (per)fluoropolyether derivatives aiming at lowering energy consumption and simplifying operations, which could provide for (per)fluoropolyether polyols having at least one end-group containing three hydroxyl groups.